Of all GNSS systems, GPS is probably the best known, and will be used as an example. Relative GPS is a known technique for providing the relative position of two objects to a greater accuracy that the absolute position of either can be found, by subtracting correlated errors from the receiver readings. Such errors include errors caused by signal delays in the ionosphere and troposphere, GPS satellite clock errors, and ephemeris errors (i.e. errors in the assumed GPS satellite positions). Differential GPS is a variant of relative GPS, where one of the objects has a known position relative to the earth.
One application of relative GPS is for landing of aircraft. This system envisages an aircraft being automatically guided through its approach and landing on a runway, using Global Positioning System (GPS) relative position measurements. The relative navigation solution required for this purpose must have a very high integrity with respect to potential failures in the system, so that the probability that the error in the relative position estimate falls outside of specified alert limits is tightly controlled.
Consequently, the technique has been extended to systems which, for reasons of integrity monitoring and robustness, collect data from several GPS ‘reference’ receivers mounted on a base station, which data is combined and encoded into a correction which can be transmitted to all aircraft and which allows for integrity monitoring. Such a system is the Local Area Augmentation System (LAAS), and reference is directed to G Xie, S Pullen et al., “Integrity Design and Updated Test Results for the Stanford LAAS Integrity Monitor Testbed”, ION 57th Annual Meeting pp. 681-693 (June 2001). These corrections can be used to partially eliminate the correlated errors from the remote vehicle measurements and thus deduce an accurate relative position. Redundancy in the data, due to the multiple reference receivers, allows integrity monitoring of the corrections before transmission. Furthermore, random errors in the base station measurements may be reduced by averaging over the reference receivers.
One method of combining GPS measurements from multiple reference receivers is based on a priori knowledge of the reference receiver locations, for example by accurate surveying. In certain situations, eg an aircraft carrier, it is not possible to determine the exact reference positions for the receivers, however techniques have been proposed whereby an initial estimate of position is used, which is subsequently cancelled out by subtraction when calculating relative positions or vectors. When the base station is mobile, the corresponding corrections allow remote vehicle position relative to the centroid of reference receivers, or relative to a central reference point, to be deduced.
Such prior art systems use code pseudorange (or carrier-smoothed pseudorange) measurements. Such measurements are amenable to the averaging process used in deriving a single set of corrections from multiple base station receivers.
It is known that greater accuracy can be achieved by using GPS carrier phase measurements, also called Accumulated Doppler (or Delta) Range (ADR). ADR measurements include an ‘ambiguity’, an unknown integer multiple of the GPS carrier wavelength. It is often possible to deduce or fix this ambiguity, by using the knowledge that it takes discrete values, which allows an increased accuracy relative position estimate.
This ambiguity in ADR measurements however, presents a problem when attempting to combine multiple measurements, and the application of the prior art code pseudorange techniques to ADR measurements has proved problematic. A further difficulty associated with the application of prior art techniques to ADR measurements is that ambiguity values are specific to satellite-receiver pairs: should the common set of satellites and reference receivers being used in a system change (for example failure of a receiver or a satellite passing out of view), ambiguity values in the corrections may also change resulting in a ‘cycle slip’.
It is therefore an object of the present invention to provide an improved method of deriving GPS correction values.